Imaging systems with high optical gain are inherently susceptible to sensor damage from high intensity radiation, whether the sensor is the human eye or a solid state device. As modern solid state detectors have become more sensitive, the susceptibility to damage from high intensity radiation has also increased. In both military and civilian environments, high intensity radiation, such as used in laser range finders, is becoming more common. Thus, there is a need for systems and methods to protect optical sensors from damage resulting from accidental or deliberate exposure to high intensity radiation.
In the prior art there are two general methods for protecting sensors from high energy radiation. A first method is to block selectively those wavelengths at which high energy radiation is encountered. This can be achieved, for example, by using dyed plastic filters in the visible spectrum or interference filters in the infrared spectrum. Dyed plastics have a limitation, however, in that they substantially reduce the out-of-band transmissions. That is, to achieve the necessary optical densities required for sensor protection, dyed filters also tend to restrict those wavelengths at which the received high intensity radiation does not exist. This limitation of out-of-band transmissions results in a significant reduction in detector sensitivity. Interference filters, on the other hand, provide substantially higher out-of-band transmission because of their narrow band filtering characteristics, but they generally have inadequate optical density when only one filter is used. Furthermore, specialized interference filters tend to be a relatively expensive components in an optical system.
A second method for protecting sensors is to use non-linear devices that are activated by high intensity radiation. Non-linear devices have advantages in that they can provide very high average transmittance when not subject to high energy radiation, and they can function over a wide range of high energy wavelengths, thus acting as broadband switches. However, activation of such devices often requires energy on the order of that sufficient to damage a sensor, which necessitates an intermediate focal plane. Furthermore, the protective range of non-linear devices can be limited by their own susceptibility to damage from high intensity radiation.
A situation to be avoided in systems designed to protect sensors from high intensity radiation is the reflection of incident radiation back toward the radiation source. In military environments where laser range finders are being used, for example, the reflection of incident radiation can have the adverse effect of revealing the presence and location of an optical system. Thus, a need exists for an optical imaging system that is transparent to normal radiation, that is opaque to high intensity radiation that would damage the sensors, and that reflects incident high intensity radiation harmlessly away from the direction of the radiation source.